My three-legged cat went missing, what can I do to find him?

Find the forth leg.

try putting up missing cat posters in the area where you live

PUT UP POSTERSIN YOUR AREA, CONTACT ANIMAL SHELTERS NEAR BY, TO SEE IF IT BEEN HANDED IN OR HURT, PUT ADS ON ANIMALS SECTIONS ON WEBSITES.

As a foster mom of a tripod, I don't know why you let him outside. But the answer is the same for a 4 legged cat. Put up posters, call your neighbors, search your neighborhood, alert your local ASPCA shelter.

Post posters around the area he went missing, and post them in local meeting place like coffee shops, libraries, etc. Always check the local shelter too.

Set up signs, call the local pet shelter, call your vet for advice